1. Field of the Invention
This invention relates, in general, to an electrophotographic copying apparatus and, more specifically, to calibration of the crosstrack and intrack platen registration edges in copiers equipped with digital scanners.
2. Description of the Prior Art
Electrophotographic copying apparatus, machines, or devices which store digital signals of the original document into memory have a scanner for digitizing the document image. Considerable random access memory is required to store the digital data for an entire page and, in some applications, several pages of a multipage original document must be stored. Because of these large memory requirements, it is desirable to store into memory only data which is needed to reproduce or duplicate the original document.
The memory requirements have, according to the prior art, been maintained at a minimum level by storing only the scanned area occupied by the document page. However, this necessitates the accurate positioning of the document page in the scanner. Most scanners have a transparent platen onto which the document is placed, manually or automatically, for scanning. Registration marks, lines, or guides are located on the platen to allow for positioning of the document in the platen area which will be scanned. By keeping the document properly aligned on the platen, and by scanning only the area within the registration marks, redundant data is kept from occupying valuable memory space.
The difficulty with such registration marks is that the scanning system must know exactly where these marks are located on the platen to conserve memory and to accurately reproduce the original properly centered on the copy paper. Once the location is determined, any change in platen position, scanning head alignment, illumination source orientation, etc., can change the location as seen by the scanner. Many times these devices are moved or changed during a service routine, and recalibration is necessary.
Therefore, it is desirable and it is an object of this invention to provide a system for easily determining the location of registration marks or guides on the platen of copying and duplicating apparatus. Further objects of the invention are to require the location process only once unless the relative position of the marks changes, and to utilize the determined location to avoid storing data into memory which does not represent the original document.